In recent years, in view of global resource conservation, improvement in fuel efficiency has been promoted for internal combustion engines. Particularly, since ignition techniques highly contribute to improvement in fuel efficiency of an internal combustion engine, various ignition techniques have been studied. A spark plug, which is a typical ignition device, is used in combination with an ignition coil for generating arc discharge in a spark discharge gap, thereby generating thermal equilibrium plasma and igniting fuel. Recently, an ignition device employing a different ignition phenomenon from a spark plug; specifically, a non-thermal equilibrium plasma ignition device, has been developed (See, for example, Japanese Patent Application Laid-Open (kokai) No. 2010-037949; Japanese Patent Application Laid-Open (kokai) No. 2014-026754; Japanese Patent Application Laid-Open (kokai) No. 2014-107198; Japanese Patent Application Laid-Open (kokai) No. 2014-123435). The non-thermal equilibrium plasma ignition device generates non-thermal equilibrium plasma by barrier discharge. Generally, barrier discharge is a discharge phenomenon in which discharge is generated by applying an AC voltage between two electrodes which are disposed with a certain gap therebetween and one or both of which are covered with an insulator. Since, different from thermal equilibrium plasma, non-thermal equilibrium plasma can be discharged in a wide space, non-thermal equilibrium plasma can efficiently generate radicals which contribute to combustion, and is thus effective for improving combustion performance.
However, the conventional non-thermal equilibrium plasma ignition plugs and the conventional non-thermal equilibrium plasma ignition devices have failed to generate a sufficient amount of plasma, resulting in a failure to sufficiently improve ignition performance. Thus, a technique for increasing the amount of generation of plasma has been desired.